Photophysical properties of hexapyrrolidine C₆₀ adducts with T_h and D₃ symmetry: Protonation of multiple basic sites

The effect of protonation on the absorption and emission properties of two isomeric hexapyrrolidine C₆₀ adducts with T_h and D₃ symmetry has been studied. Results show that protonation does not modify substantially the interesting emissive properties reported in previous work for the neutral forms. The absorption and fluorescence spectra, as well as the fluorescence decay, are insensitive to protonation at any acid concentration for the D₃ isomer, while small changes are seen for the T_h isomer. For the latter, as the positive charge of the molecule increases the absorption spectrum shifts to lower wavelengths, the emission spectrum shifts to the red, and the fluorescence lifetime increases. The behavior of this isomer upon acid addition can be explained on the basis of a complex equilibrium between species having different degrees of protonation. Results show that the equilibration rates among the various species in the excited state are slower than the fluorescence decays, and consequently the measured decay is a weighted average decay of the individual species. The surprising difference in the behavior of the D₃ isomer was shown not to originate from significant differences in the basicity of the nitrogen atoms, but probably due to differences in the polarizability of the two π-systems.